Mechanisms of influence of gas pressure on isotopically selective multiphoton dissociation of CF3Br

Experimental and theoretical studies were made of mechanisms of the influence of the gas pressure on isotopically selective multiphoton dissociation of CF₃Br by pulsed CO₂ laser radiation. The dissociation yields and selectivity were determined at three different frequencies [9P(18), 9P(24), and 9P(32) lines of a CO₂ laser] corresponding to preferential excitation of ¹³CF₃Br. A simple four-level model is proposed to describe these dependences and take into account rotational relaxation and intermolecular vibrational exchange. As a result of these processes, the rotational and vibrational bottleneck is overcome. A sharp drop in the dissociation selectivity when a specific gas pressure is exceeded is due to an explosive increase in the multiphoton dissociation yield of the nonresonant isotopic component (¹²CF₃Br) due to its vibrational self-heating.